In the spirit of this post, I'll add my own disclaimer here: I am not a lawyer, and this is not legal advice. If you have any legal questions about licensing, consult a lawyer.
Licensing is an important part of open-source software. It is the way that you, as the author, can control how your software is used and modified. It is also the way that you can protect yourself from liability. Licensing is one of the darker corners of the FOSS ecosystem, and let's face it, legal stuff is boring. But it's important, so let's get through it.
There's more to publishing open-source code than uploading it to GitHub. While GitHub's terms of service allow others to view and fork public projects, you are still the copyright holder. Copyright is given to the author of an original work under most jurisdictions. We assume you are the author and have the right to license and distribute the code.
If you upload your code to a public site without a stated license, you maintain the usage and distribution rights, but you are not granting any rights to others. This means that others can view your code, but if they execute or distribute it they are technically transgressing your rights. If you're in the habit of using or distributing unlicensed code from GitHub projects, beware. The author could have grounds for a civil copyright violation suit against you. Unlicensed code on the web is not open-source code. So it's a bad idea to use unlicensed code yourself, and if you want people to use and contribute to your project, you need to license it.
Another motivation for licensing your code is to protect yourself from liability. If you are distributing code that others can use, you need to clarify that you are not responsible for any damage it may cause. If you write code intended for safety critical systems, you should probably consult a lawyer. But for most of us, a simple disclaimer is enough.
